1. Field of the Invention
The invention pertains to the field of fittings and adaptors that can be used to secure insulated electrical cables for the transmission of signals. More specifically, the fittings and adaptors are used to retain cables that are shielded from the effects of electromagnetic disturbances.
2. Discussion of the Related Art
Cables for use in transmitting electronic signals are often provided with shielding in the form of foil, wire mesh or screen material that surrounds one or more central insulated leads. One common example of this type of cable is the coaxial cable that is used to carry television or data transmissions. The shielding itself is usually covered with an external layer of insulation. The shielding protects the wanted signal that is being transmitted on the central lead from ambient electromagnetic disturbances. The shielding also limits the amount of electromagnetic disturbance that is transmitted outwardly from the central lead. For these reasons, shielded cables are increasingly utilized in densely packed arrays of electrical equipment.
A variety of connectors are used to secure and interconnect these cables. Typically, bulkhead connectors, which connect the shielding to a ground at the terminus of the cable, are used for shielded cables. Nevertheless, it is not always desirable or practical to connect the cable to ground at its terminus. For example, U.S. Pat. No. 5,975,953 to Peterson describes the difficulties and special considerations that are involved when connecting electromagnetic interference (EMI) shielded cables directly to an input/output (I/O) card and having to shunt the ground path through the I/O card.
Further, in the case of bulkhead connectors, a continuing problem exists with securing the cables against unwanted motion that can, for example, cause signal degradation by torsional or translational motion of the cable. Prior systems are unable to secure the cable against unwanted motion while providing a ground for the EMI shielding. Separate structures, such as a rubber grommet that is separate from the bulkhead connector are required for these purposes.
Regulatory agencies are promulgating ever stricter regulations that increasingly limit the amount of EMI that electronic equipment may generate. Additionally, stricter EMI limits are necessarily imposed by the practicalities of operating computer and telecommunications systems at increasingly faster rates of data transmission.
The cable retention device that is shown and described herein overcomes the aforementioned problems and advances the art by providing a shielded cable retention device that provides an EMI ground connection at a location other than the cable terminus. The cable retention device is particularly effective in providing shield protection at a point where the cable passes through a chassis that may house, for example, a computer or other electrical equipment making use of shielded cables.
A cable retention device according to the preferred embodiments described herein comprises a conduit having an exterior wall operably configured to receive the shielded cable and an end plate having an aperture. The conduit presents an exterior surface. An end plate defines an aperture, and the conduit is joined with the end plate at the aperture so that the conduit""s passageway is continuous through the end plate. A clamp, such as a metal band, has dimensions compatible with the conduit""s exterior surface. The clamp circumscribes a portion of the conduit""s exterior surface and retains a piece of the cable shielding between the clamp and the portion of the exterior surface, thus grounding the shielding to the conduit when the conduit is made of a conductive material establishing a pathway to ground.
The conduit may be any tubular member, such as a cylinder or square tube, or may be non-encapsulating, such as a channel. The conduit preferably but optionally presents an axis of symmetry, and the end plate may comprise at least one male boss protruding transversely with respect to the axis of symmetry. In this case, the end plate preferably also comprises a female slot located distal from the male boss. The female slot has compatible dimensions for receipt therein of members of identical size in respect to the male boss when such members are available for receipt within the female slot. This arrangement of bosses and slots permits identical units formed of conduits and end plates to be stacked atop one another where the slots and bosses locate or index the respective units in fixed positional relationship with respect to one another. The conduit and the end plate may be joined by any mechanism, for example, including press fits, integral formation of parts, or a bonding composition such as an adhesive or solder.
Especially preferred end plates present a rearward face forming a plane in transverse orientation with respect to the axis of symmetry of the conduit. The end plate comprises an alignment ear protruding beyond the rearward face, which serves to locate or index each of the end plate-cylinder units with respect to a chassis or other mounting structure.
In other preferred but optional embodiments, the end plate comprises a shoulder having a fulcrum. A guide rail mounting bracket has an opening of sufficient size to permit passage of the conduit while not permitting passage of the shoulder. The guide rail preferably has a first end and a second end distal from the first end. The first end comprises a mounting tab for use in retaining the guide rail in complimentary mounting structure on the chassis when the guide rail is installed in the complimentary mounting structure. The second end comprises an opening with a threaded fastener received therein for use in demountably attaching the guide rail to the complimentary mounting structure when the guide rail is installed in the complimentary mounting structure. The fulcrum is used in flexing the guide rail under force exerted by the threaded fastener when the guide rail is installed in the complimentary mounting structure.
The aforementioned structure including a conduit and an end plate that are joined to present a continuous passageway, facilitates a method of retaining a shielded cable. The method comprises the steps of clamping the end plate in a bracket or guide rail, removing insulation from the insulated and shielded cable to expose shielding in the cable, positioning the shielding around the conduit, and clamping the shielding to the conduit. The section of cable that resides within the cable is, thus, not crimped or pinched in a manner that could cause signal degradation.